Polymer electrolyte fuel cells are required to have high conductivity and excellent current collection capability while exhibiting mechanical strength to withstand various operations. Also, the diffusion of substances that contribute to electrode reactions needs to be excellent in such fuel cells. To respond to such requirements, carbonized sheets are generally used as electrode substrates. Fuel cell applications that have attracted attention in recent years are those for automotives where high power density is required. In such applications fuel cells are operated in regions of high current density, and amount of water generated per unit reaction area increases. Accordingly, efficient discharge of water produced by reactions is the issue, and high dewatering capability is thereby required for carbonized sheets used as gas diffusion materials in fuel cells. Therefore controlling pore distributions in the electrode substrate has been attempted for the enhancement of dewatering capability.
For example, the objective of Patent Literature 1 is to provide a porous carbon material suitable for making electrodes such as those having high mechanical strength and excellent electrical characteristics and having a unimodal pore distribution with a clear peak. On the other hand, in Patent Literature 2, the target is set to have two types of pores: one, pores mechanically pierced in a sheet, and the other, voids among fibers of a non-woven fabric. However, neither is sufficient to achieve both mechanical strength and dewatering capability.
In addition, Patent Literature 3 introduces a method for blending powder of graphite, carbon black and the like. However, only a peak of those with a pore diameter of no larger than 1 μm is observed in the pore distribution, and thus no significant improvement in dewatering capability is achieved. Furthermore, Patent Literature 4 introduces a method for laminating layers formed under different press-molding conditions so as to obtain a porous carbon sheet having different pore distributions in a thickness direction. However, the sheet tends to warp due to different upper- and lower-side structures. In addition, those literatures do not clearly indicate how to set pore distributions for improving dewatering capability and enhancing fuel cell performance.
Patent Literature 5 provides cell performance testing that is not described in Patent Literatures 1-4: that is, by setting bimodal (two-peak) pore distributions, cell performance is enhanced over that of conventional unimodal pore distributions.
In automotive applications, it is required to maintain internal environment of a fuel cell stable under a wide variety of conditions; not only high power density conditions that corresponds to pressing down on the accelerator but also low power density conditions that corresponds to traveling at a constant speed. Namely, power must be generated in the presence of residual water under low temperature conditions such as at the startup of the fuel cell, while also be generated under high-temperature and wet conditions after the accelerator was pressed down on. Using a method described in Patent Literature 5, cell performance is improved only when power generation conditions are relatively constant such as in stationary applications, but no description is provided for different conditions. Accordingly, the method is not suitable for automotive applications.
Meanwhile, as methods for continuous sheet molding, intermittent pressing described in Patent Literatures 3 and 4, and double-belt pressing (DBP) described in Patent Literatures 5 and 6 are widely known. According to those literatures, in order to control the thickness, using a spacer or cotter is preferred at a part where pressure is applied and thickness is determined. However, no description is provided for controlling pore distributions in such a process.
The problem factor leading to a decrease in cell performance is insufficient gas supply caused by a clog in the substrate or separator passages, also known as flooding or plugging, that may instantaneously decrease power generation capability in a relatively low-temperature high-current-density region. On the other hand, under high-temperature and dry conditions, cell performance may be lowered by a decrease in proton conductivity as the electrolyte membrane dries, also known as a dry-up phenomenon.
Considering the above phenomena from the viewpoint of porous electrode substrates, conventional mainstream porous electrode substrates, either paper or cloth type, show a highly symmetrical pore distribution peak. Namely, pore diameters are substantially uniform in the entire substrate, and it is not clear what routes are taken by the fuel gas and reaction-produced water to enable the gas to diffuse and pass through. Accordingly, when a route is clogged by the produced water, gas diffusion is blocked, and drying accelerates once it starts, thereby resulting in a dry-up phenomenon.
Considering those problems, what are desired are porous electrode substrates adaptable to a wide range of fuel cell conditions from low-temperature and wet conditions to high-temperature and dry conditions.